1. Field of the Invention
The present invention relates to a high-frequency magnetic field generator/detector and, more particularly, to a type adapted for use in a nuclear magnetic resonance apparatus.
2. Description of the Prior Art
Out of the conventional devices of such type, there is known an example of FIG. 1.
FIG. 1 is an equivalent circuit diagram of a conventional high-frequency magnetic field generator/detector described in David G. Gadian, "Nuclear Magnetic Resonance and its Application to Living Systems", Oxford University Press, p. 170. In the diagram are shown an inductance 1 of a high-frequency coil, a resistance 2 thereof, impedance-matching variable capacitors 3a and 3b, and an input-output terminal 4 connected to a high-frequency transmitter/receiver (not shown).
In the above configuration, its operation is performed in the following manner.
The impedance of a high-frequency coil is generally different from the input-output impedance of a high-frequency transmitter/receiver (not shown) which is connected to such known device for detecting the generation of a high-frequency magnetic field, so that proper impedance matching is necessary therebetween by means of variable capacitors 3a and 3b to maximize the power transmission efficiency.
In the circuit diagram of FIG. 1 where L and r are an inductance and a resistance of the high-frequency coil, respectively: static capacitances C and C' of the variable capacitors 3a and 3b are expressed as ##EQU1## where R is the input-output impedance of the high-frequency transmitter/receiver and .omega. is an angular frequency for achieving the above impedance matching.
In an exemplary case of effecting the impedance matching at an angular frequency of 40 MHz as .omega.=2.pi..times.40.times.10.sup.6 sec.sup.-1, the values of C and C' are obtained as follows when L=1.times.10.sup.-6 (H), r=1 (.OMEGA.) and R=50 (.OMEGA.): EQU C.apprxeq.14 (pF) EQU C'.apprxeq.2 (pF)
Voltages Vc and Vc' applied to the capacitors 3a and 3b respectively are expressed as ##EQU2## where P is a high-frequency input power. The values of Vc and Vc' are obtained as follows when .omega.=2.pi..times.40.times.10.sup.6 sec.sup.-1, L=1.times.10.sup.-6 (H), P=1000 (W) and r=1 (.OMEGA.): EQU Vc.apprxeq.Vc'.apprxeq.8000 (V)
Since the conventional high-frequency magnetic field generator/detector is so constituted as mentioned above, the capacitances of the variable capacitors employed for impedance matching become extremely small and particularly the value of C' is on the order of 1 pF, so that designing the matching circuit is rendered difficult and satisfactory stability is not attainable.